Fall 2011

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark

December 6, 2011

The sycamore: Tall, pale and thin-skinned

After the leaves have dropped, the ghostly white trunks and limbs of the American sycamore visually leap out from the somber timber around them.

Sycamore bark is smooth and pale because the trees exfoliate their bark in the summer. As branches thicken, their brittle, tan outer bark splits into pieces that curl up and flake off, revealing a gray, white or greenish smooth inner bark. Several theories suggest how sycamores might benefit from bark shedding:

Shucking old bark could help jettison pests clinging to the bark.

A thin bark might allow the fast-growing tree to release water more efficiently during transpiration, which would accelerate growth. The tree's smooth inner bark can turn green with chlorophyll and continue to manufacture carbohydrates in the autumn and early spring while the tree is leafless.

Roots absorb oxygen necessary for respiration in the tree's cells, but sycamores often grow near waterways, where water-saturated, oxygen-poor soils are common. Compared with thicker bark, a thin bark might be better able to absorb oxygen and help counter the roots' absorption deficit.

Sycamores thrive in urban settings, where their rapid growth, wind resistance and tolerance of pollution and compacted soils make them good street trees. American sycamores, however, are vulnerable to anthracnose infections, so urban arborists tend to favor planting disease-resistant American-Eurasian sycamore hybrids, such as the London planetree.

SOURCES: New York City Dept. of Parks and Recreation, U.S. Forest Service, Ohio State University, American Midland Naturalist, Casey Trees, Colorado State University